The boron-coated straw (BCS) detector is based on arrays of thin walled boron-coated copper tubes. The elemental component of this detector is a long tube (“straw”), generally about 1 to 4 mm in diameter, coated on the inside with a thin layer of 10B-enriched boron carbide (10B4C). Thermal neutrons captured in 10B are converted into secondary particles, through the 10B(n,α) reaction:10B+n→7Li+α  (1)
The 7Li and α particles are emitted isotropically in opposite directions with kinetic energies of 1.47 MeV and 0.84 MeV, respectively (dictated by the conservation of energy and momentum). For a boron carbide layer that is only about 1 μm thick, one of the two charged particles will escape the wall 78% of the time, and ionize the gas contained within the straw.
Each BCS detector is operated as a proportional counter, with its wall acting as the cathode, and a thin wire tensioned through its center serving as the anode electrode, operated at a high positive potential. Primary electrons liberated in the gas drift to the anode, and in the high electric field close to the anode, avalanche multiplication occurs, delivering a very much amplified charge on the anode wire. Standard charge-sensitive preamplifier and shaping circuitry are used to produce a low noise pulse for each neutron event. Gamma interactions in the wall produce near minimum ionizing electrons that deposit a small fraction of the energy of the heavily ionizing alpha and Li products. Gamma signals are effectively discriminated with a simple pulse height threshold.
Applicant has previously published articles on BCS detection capabilities, fabrication, and development of prototypes for various applications including:    J. L. Lacy, A. Athanasiades, N. N. Shehad, R. A. Austin, C. S. Martin, “Novel neutron detector for high rate imaging applications”, in IEEE Nuclear Science Symposium Conference Record, 2002, vol. 1, pp. 392-396;    A. Athanasiades, N. N. Shehad, C. S. Martin, L. Sun, J. L. Lacy, “Straw detector for high rate, high resolution neutron imaging”, in IEEE Nuclear Science Symposium Conference Record, 2005, vol. 2, pp. 623-627;    J. L. Lacy, A. Athanasiades, N. N. Shehad, C. S. Martin, L. Sun, “Performance of 1 Meter Straw Detector for High Rate Neutron Imaging”, in IEEE Nuclear Science Symposium Conference Record, 2006, vol. 1, pp. 20-26;    J. L. Lacy, A. Athanasiades, C. S. Martin, L. Sun, T. D. Lyons, “Fabrication and materials for a long range neutron-gamma monitor using straw detectors”, in IEEE Nuclear Science Symposium Conference Record, 2008, pp. 686-691;    A. Athanasiades, N. N. Shehad, L. Sun, T. D. Lyons, C. S. Martin, L. Bu, J. L. Lacy “High sensitivity portable neutron detector for fissile materials detection”, in IEEE Nuclear Science Symposium Conference Record, 2005, vol. 2, pp. 1009-1013;    J. L. Lacy, A. Athanasiades, C. S. Martin, L. Sun, J. W. Anderson, T. D. Lyons, “Long range neutron-gamma point source detection and imaging using unique rotating detector”, in IEEE Nuclear Science Symposium Conference Record, 2007, vol. 1, pp. 185-191;    J. L. Lacy, L. Sun, C. S. Martin, A. Athanasiades, T. D. Lyons, “One meter square high rate neutron imaging panel based on boron straws”, in IEEE Nuclear Science Symposium Conference Record, 2009, pp. 1117-1121; and    J. L. Lacy, A. Athanasiades, L. Sun, C. S. Martin, G. J. Vazquez-Flores, “Boron coated straw detectors as a replacement for 3He”, in IEEE Nuclear Science Symposium Conference Record, 2009, pp. 119-125.    J. L. Lacy, L. Sun, A. Athanasiades, C. S. Martin, R. Nguyen, and T. D. Lyons, Initial performance of large area neutron imager based on boron coated straws. IEEE 2010 Nuclear Science Symposium Conference Record, (2010) pp. 1786-1799.    J. L. Lacy, L. Sun, C. S. Martin, R. Nguyen, A. Athanasiades, and Z. Sobolewski, Initial performance of sealed straw modules for large area neutron science detectors. IEEE 2011 Nuclear Science Symposium Conference Record, (2011) pp. 431-435.    J. L. Lacy, A. Athanasiades, L. Sun, C. S. Martin, T. D. Lyons, M. A. Foss, and H. B. Haygood, Boron-coated straws as a replacement for 3He-based neutron detectors. Nuclear Instruments and Methods in Physics Research A, vol. 652 (2011), pp. 359-363.These references are hereby incorporated by reference into this application in their entirety for all purposes.
Additionally, Applicant is the inventor of several patents and patent applications related to boron-coated straw detectors including:                U.S. Pat. No. 7,002,159 entitled “Boron-Coated Straw Neutron Detector”;        U.S. Pat. No. 8,330,116 entitled “Long Range Neutron-Gamma Point Source Detection and Imaging Using Rotating Detector”;        U.S. Pat. No. 8,569,710 entitled “Optimized Detection of Fission Neutrons Using Boron-Coated Straw Detectors Distributed in Moderator Material”;        U.S. patent application Ser. No. 13/106,818 filed May 12, 2011, entitled “Neutron Detectors for Active Interrogation”; and        U.S. patent application Ser. No. 13/106,785 filed May 12, 2011, entitled “Sealed Boron-Coated Straw Detectors in Moderator Material.”These patents and pending applications are hereby incorporated by reference in their entirety for all purposes.Escape Efficiency        
In order for neutrons stopped in the straw array to be detected, the decay fragments must escape the thin layer of 10B4C in each straw. The escape probability can be derived from the solid angle formed between the point of neutron interaction and the exit interface, and is written as:
                                                                                          ɛ                  esc                                =                                ⁢                                  1                  -                                      T                    ⁢                                          /                                        ⁢                                          (                                              4                        ⁢                                                  L                          α                                                                    )                                                        -                                      T                    ⁢                                          /                                        ⁢                                          (                                              4                        ⁢                                                  L                          Li                                                                    )                                                                                  ,                                                for                  ⁢                                                                          ⁢                  T                                ≤                                  L                  Li                                                                                                                        =                                ⁢                                                      1                    ⁢                                          /                                        ⁢                    2                                    +                                                            L                      Li                                        ⁢                                          /                                        ⁢                                          (                                              4                        ⁢                        T                                            )                                                        -                                      T                    ⁢                                          /                                        ⁢                                          L                      α                                                                                  ,                                                for                  ⁢                                                                          ⁢                                      L                    Li                                                  <                T                ≤                                                      L                    α                                    (                                      8                    ⁢                    b                                    )                                                                                                                        =                                ⁢                                                      (                                                                  L                        α                                            +                                              L                        Li                                                              )                                    ⁢                                      /                                    ⁢                                      (                                          4                      ⁢                      T                                        )                                                              ,                                                for                  ⁢                                                                          ⁢                  T                                >                                                      L                    α                                    (                                      8                    ⁢                    c                                    )                                                                                        (                  8          ⁢          a                )            where T is the film thickness, and Lα and LLi are the ranges of the α and 7Li, respectively, inside the 10B4C film, equal to Lα=3.30 μm and LLi=1.68 μm. The ranges were computed in SRIM-2006.02 (http://www.srim.org/) for a target layer of 10B4C with a density of 2.38 g/cm3 and for ion energies of 1.47 MeV for alphas and 0.84 MeV for 7Li. The escape efficiency computed here is slightly underestimated, because for simplicity we only considered the dominant branch of the 10B(n,α) reaction. The other branch (6% of cases) generates more energetic products, which have slightly better chances for escape. Equation (8) has been evaluated for T values up to 10 μm, and is plotted in FIG. 8. For a 10B4C film thickness of 1.0 μm, the escape efficiency is 78%.